1. Field of the Invention
This invention relates to piston and cylinder type power cylinders in general and to fluted piston and rod assembly in particular, for permitting safe operation of the cylinder subjected to both internal pressure hoop stress and to external side loading in service, in particular when injection molded economically from engineering plastics for use with gas cylinders.
2. Description of the Prior Art
At the present time the conversion of all metal power cylinders, in particular high pressure gas cylinders used for assisting the movement of structural members of scientific equipment, including table tops, chair seats, operating tables or other members, to low pressure devices for safer operation using newly introduced materials such as engineering plastics which by nature are not as strong as metals but more attractive economically requires drastic design changes to sustain physical and operational abuse with equal success.
As a general rule in power cylinders, the size of a piston rod is only a fraction of the piston to which it is normally secured by welding or threaded connection when produced from metal. In order to closely balance directional end forces of a double acting power cylinder, the pressure of the working fluid over the piston sides must be varied accordingly in proportion to the areas provided. As a result, cylinders with heavier piston rods require pressure increase to the piston underside for return from rod extended to rod retracted position with equal force. For applications which do not employ pressure force for rod retraction, in particular in structures such as support columns used widely as single legs in chairs wherein the seat may be mounted upright to the piston rod while the cylinder is fixed to the base, the weight of a person may act as an external force for rod retraction from the extended position. An increase in piston rod diameter may be exploited here to the advantage using lower operating pressures. This is so because the end force to the piston face inside tubular housing with rod protruding a portion subjected to atmospheric or ambient conditions is equal to the area of the rod times internal pressure. In turn, devices such as gas cylinders which operate on a charge of pressurized working fluid circulating internally between piston sides via fluid passages incorporated therein and controlled externally during unit operation can be produced from material other than metal. For example, metal piston rod of 1/4 inch diameter and 0.5 inch squared cross section adaptable for carrying 100 lbs axial load would require pressure of 2000 psig inside cylinder. For the same end load of 100 lbs a piston rod of 11/8" diameter having 1 inch.sup.2 cross section requires only 100 psig pressure resulting in lower stresses and safer operation. The advantages offered by the tradeoff in rod diameter allow consideration of more economical materials such as plastics even if it is not as strong as the metal found in conventional cylinders.
The designs incorporating fluted piston into the cylinder can revolutionize conventional concepts. No doubt the economics would be prohibitive to construct a metal piston with flutes. In particular that using metal piston rod of small diameter which can carry an ample end load when attached to a piston of generally much larger diameter, piston sides provide practically equal fluid chambers when piston position inside the cylinder changes from a rod extended, creating a first fluid chamber in cylinder end facing piston head, to a rod retracted position, creating a second fluid chamber in the opposite cylinder end facing piston underside. This is not so when the rod diameter is greatly increased to carry the same load while using plastics. The second cylinder end facing piston underside, while housing larger piston rod, would result in the second fluid chamber of considerably smaller and practically unacceptable volume from operational standpoint. This is so because the cylinder, pre-charged with an initial pressure of say 100 psig is forced to circulate fluid from the first chamber which is larger to a second chamber which is smaller. As a result, when a larger external force is applied to the piston rod end for piston position change, the internal gas pressure inside second fluid chamber increases in proportion to ratio of volumes chambers provide. The cylinder will not operate properly unless piston is elongated and provided with flutes for volume compensation. Elongated piston with flutes, cut slightly below piston rod diameter, serves not only as volume compensator between fluid chambers, but also doubles as an elongated bearing surface in contact with cylinder bore to handle side loads of the unit in a specific application, in particular in chairs subject to height adjustment.
In turn, cylinders with fluted novel piston produced by injection molding from plastics yield economy not attainable by fabricated metal cylinders, advancing the state of the art of such devices beyond anticipation.